Generally, a two-wire transmitter receives power-supply voltage supplied via a loop signal line, and outputs an analog DC current signal of 4-20 mA according to, e.g., a measurement value. In recent years, a hybrid communication method for transmitting a digital signal superimposed on an analog DC signal as typified by a highway addressable remote transducer (HART) communication method has been broadly used. Bidirectional communication of the digital signal is performed so that transmitter status information can be acquired and a transmitter parameter can be set, for example.
As illustrated in FIG. 3, in a two-wire transmitter 210 employing such a hybrid communication method, connection with a wireless device 220 is made, an analog signal is transmitted via a loop signal line, and a digital signal is wirelessly transmitted/received via the wireless device 220. Accordingly, even the two-wire transmitter 210 having no wireless communication function can perform digital signal wireless communication.
In an example of this figure, the wireless device 220 includes a built-in communication operation battery. Among the wireless devices 220 with the built-in batteries, one having the function of supplying power of the built-in battery to the two-wire transmitter 210 has been also in practical use.